1. Field
The disclosed concept relates generally to surge protection devices, and in particular to a surge protection apparatus having multiple surge protections devices and a display which indicates which one of the surge protections devices, if any, has experienced a failure.
2. Background Information
Electrical systems, such as electrical power distribution systems, periodically experience over-voltage conditions, such as transient over-voltage conditions, also called “surges.” Over-voltage conditions are problematic to electrical systems because they may cause damage to the loads, such as electronic devices or other hardware, that are coupled thereto. As a result, surge protection devices (SPDs) have been developed to protect the loads from over-voltages that would otherwise damage the loads. SPDs typically provide such protection by coupling various types of known transient-suppressing elements between the phase, and neutral and/or ground conductors of an electrical power distribution system.
As is known in the art, transient-suppressing elements, such as metal-oxide varistors (MOVs), silicon avalanche diodes (SADs) and gas discharge tubes (GDTs), typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a predetermined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and “shunting” the current associated with the over-voltage through the transient-suppressing element and away from the load.
MOVs are probably the most commonly used transient-suppressing element. An MOV consists of two plates separated by an insulator, such as a metal oxide, that has a known voltage breakdown characteristic. When the voltage between the two plates reaches a certain level (the voltage breakdown level), the insulator breaks down and conducts current. MOVs, however, have operational limitations that must be taken into account when designing an SPD. Specifically, all MOVs have a maximum surge current rating that, if exceeded, may cause the MOV to fail. An MOV may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of the transient current conducted by the MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure.
In light of these limitations, prior art SPDs have been developed that use multiple MOVs in a parallel combination such that the MOVs share the total transient current. Each individual MOV in such a configuration only conducts a portion of the total transient current, making it less likely that any individual MOV will exceed its maximum transient current capacity. In addition, an SPD that uses a plurality of parallel MOVs can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV. If internally fused and sorted by V/I characteristics, a parallel combination of MOVs is advantageous because the failure of any individual MOV will not cause a complete loss of SPD functionality.
There is room for improvement in the field of SPDs.